1. Field of the Invention
The present invention relates to a thin-film resistor, and more particularly, it relates to a thin-film resistor provided with a highly reliable thin-film nitride resistance member whose resistance value is not substantially changed under high temperature conditions.
2. Description of the Prior Art
A thin film comprising nitrides of elements belonging to groups III-VI of the periodic table such as tantalum nitride, titanium nitride, zirconium nitride, hafnium nitride, aluminum nitride, niobium nitride, boron nitride and chromium nitride is known to be stable under high temperature conditions and to be excellent in electrical characteristics. A highly reliable thin-film resistance member of a precision type having a small resistance temperature coefficient may be formed from one of these nitrides or from a combination of two or more such nitrides. Also, a thin film comprising nitrides of elements belonging to groups VII and VIII of the periodic table such as Mn.sub.2 N, Mn.sub.3 N.sub.2, Mn.sub.4 N and Fe.sub.2 N, Fe.sub.4 N, CoN, Co.sub.2 N, Co.sub.3 N.sub.2, Ni.sub.3 N and Ni.sub.3 N.sub.2 is known to be stable under high temperature and excellent in electric characteristics.
Such a thin-film nitride resistance member is formed on an insulating substrate of glass, ceramic material, etc. by a method such as electron beam deposition, ion beam deposition, flash deposition, cathode sputtering deposition and the like. Such a thin-film resistance member can also be formed by hot press, sublimate recrystallization, discharge reaction or chemical vapor deposition. In general, such thin-film resistance members are usually formed through reactive sputtering deposition performed in an atmosphere of high-purity nitrogen gas and high-purity argon.
The thin-film nitride resistance member is provided thereon with an electrode for external connection, which comprises a multi-layer electrode of Cr-Cu, Cr-Au, Ni-Cu, Ni-Au, Ni-Ag, NiCr-Au, Ti-Pd-Au, Ti-W-Au and the like. In an external connection electrode having a multi-layer structure, a first layer of Cr, Ni, NiCr or Ti serves as an adhesion layer for the thin-film nitride resistance member and an outer layer of Cu, Au or Ag serves as a solderable layer.
Such a resistor provided with a thin-film nitride resistance member shows no change in characteristics in lifetime tests such as a moisture-resistance loading test at the room temperature. However, tests have been performed in which the resistance value of such a resistor was changed when the same was held at a high temperature of, e.g., 150.degree. C. or subjected to a rated voltage loading test at 70.degree. C. Such a phenomenon was observed in resistors both coated and not coated with insulating resin and also in a hermetically sealed one, and the resistance values were changed at equal rates.
This means that the resistance films were changed under high temperature conditions. In an effort to find the cause thereof, it has been proved that the resistance value of such a thin-film nitride resistance member is changed because nitrogen contained in the resistance film is partially dissociated in a contact region between the resistance film and the external connection electrode under high temperature conditions, the nitrogen being transferred to the metal forming the electrode. When, for example, a resistor comprising a thin-film nitride resistance member of zirconium nitride (ZrN) and an external connection electrode formed with a first layer of NiCr and a second layer of Au is held at a temperature of 150.degree. C., the color tone of the zirconium nitride thin film is changed with time in the vicinity of the external connection electrode, from brown to colorless transparency. Such a phenomenon has been analyzed by means such as ESCA and EMX, and it has been found that nitrogen contained in the zirconium nitride thin film is gradually dissociated and transferred to the NiCr in the external connection electrode, causing the color change of the resistance film as well as a change in resistance value.
In other words, the following reaction is caused in the contact portion between the thin-film resistance member and the metal of the external connection electrode: EQU Me.sup.I N+Me.sup.II .fwdarw.Me.sup.I N.sub.I-X +Me.sup.II N.sub.X
(Me.sup.I N: thin-film nitride resistance member; Me.sup.II : external connection electrode)
This is because the external connection electrode is made of metal, which traps nitrogen contained in the thin-film nitride resistance member upon application of a high temperature so as to nitrogenize the electrode.